Recently, as depletion of conventional energy resources such as oil or coal is foreseen, interest in alternative energies is increasing. A fuel cell is one of the alternative energies, and advantageously has a high efficiency, does not emit pollutants such as NOx and SOx and uses a fuel that is abundant in quantity, and therefore, the fuel cell attracts public attention.
The fuel cell is a power generation system that converts chemical reaction energy of a fuel and an oxidant to electrical energy, and typically hydrogen and hydrocarbon, for example methanol or butane, are used as a fuel, and oxygen is representatively used as an oxidant.
In the fuel cell, a membrane electrode assembly (MEA) is the basic unit for generating electricity, and includes an electrolyte membrane and anode and cathode electrodes formed at opposite sides of the electrolyte membrane. FIG. 1 illustrates the principle for generating electricity of a fuel cell, and Chemical Figure 1 represents a reaction formula of a fuel cell in the case that hydrogen is used as a fuel. Referring to FIG. 1 and Chemistry Figure 1, an oxidation reaction of a fuel occurs at an anode electrode to generate hydrogen ions and electrons, and the hydrogen ions move to a cathode electrode through an electrolyte membrane. The hydrogen ions transmitted through the electrolyte membrane and the electrons react with oxygen (oxidant) at the cathode electrode to generate water. This reaction causes the electrons to move to an external circuit.
Chemistry Figure 1
Anode electrode: H2→2H++2e−
cathode electrode: ½O2+2H++2e−→H2O
Reaction formula: H2+½O2→H2O
FIG. 2 illustrates a general configuration of a membrane electrode assembly for a fuel cell. Referring to FIG. 2, a membrane electrode assembly for a fuel cell includes an electrolyte membrane 201, and an anode electrode and a cathode electrode located at opposite sides of the electrolyte membrane 201. The anode and cathode electrodes respectively include catalyst layers 203, 205 and a gas diffusion layer 208. The gas diffusion layer includes electrode substrates 209a, 209b and microporous layers 207a, 207b formed on the electrode substrates.
Studies for a fuel cell exhibiting a high power required to enhance compatibility of a fuel cell with many advantages as mentioned above become more active, and particularly the demands on fuel cells capable of continuously providing high power are more increased.
A fuel cell generates electricity by moving hydrogen ions as mentioned above. Here, what is helpful for moving such hydrogen ions is moisture, which is also a resultant of reaction in the electrode. However, the amount of moisture generated as a result of reaction is not so sufficient to fully ensure ion conductivity of a fuel cell, so a fuel cell is generally operated under a humidity condition.
However, if there exists an excessive amount of moisture, flooding occurs, which may block fine holes in a catalyst layer or a gas diffusion layer, decrease a three-phase reaction point, and decrease the efficiency of a fuel cell resultantly.
As mentioned above, the amount of moisture in an electrode of a fuel cell is a factor dominating the performance of the electrode. Thus, in order to obtain a high power from a fuel cell, moisture introduced into an electrode or generated from the electrode should be suitably controlled.
However, such control is very troublesome, and no effective solution has been suggested up to now. Thus, it is urgent to develop a technique capable of maintaining a suitable amount of moisture in an electrode of a fuel cell to obtain a high power.